Two-Dimensional Models of Poroelastically-Controlled
Earthquake Triggering
Permeability is the primary control on the nature of fluid flow within mid-ocean
ridge hydrothermal systems. This parameter determines the intensity of hydrothermal
convection, the spatial form of circulation patterns, and the location and extent of
downflow and upflow zones. Permeability plays a key role mediating many subseafloor
processes, including fluxes of heat and chemicals, geochemical alteration, and
primary production by chemosynthesizing microorganisms. Despite this importance,
permeability is the most poorly constrained of the hydrological parameters in
mid-ocean ridge settings.
I have developed a new technique for constraining the permeability structure in
these systems using numerical models of poroelastic fluid flow. These models are
calibrated using results from seismic studies that show spatial patterns in the
tidal triggering of mid-ocean ridge earthquakes. The spatial triggering patterns
likely result from pore pressure transients, which are generated by tidal forces and
influenced by permeability structure. These pressure transients diffuse through the
crust creating "waves" of enhanced seismicity by lowering normal stresses on fault
surfaces. Relative phase lags in stresses associated with these pressure transients
can be modeled to approximate the stresses inferred from earthquake data. These
models provide constraints on the permeability structure of young oceanic crust.
Preliminary results using a microearthquake catalog from the East Pacific Rise show
that the phases of volumetric stresses inferred from the earthquake data can be
approximated using realistic permeability values and simple permeability
distributions. These models predict that the average permeability in the upper 1.5
km of crust in the along-axis direction is about 10-12 m2,
which is consistent with estimates based on heat flow data. Future work using
forward modeling techniques will provide new insights into the permeability
structure of seafloor hydrothermal systems.
Animations
Animation showing pressure perturbations within the model domain resulting from
ocean tidal loading and Earth tide stresses. Pressure perturbations are dominated by
ocean tidal loading:
Animation showing effective stress perturbations for the poroelastic frame within
the model domain. Effective stresses are dominated by the Earth tides, but are
strongly mediated by pore pressure perturbations:
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